The present invention relates to improving the quality of products produced by plastic resin extrusion lines and to the production of blown film.
When blown film is extruded, it typically is in the form of a continuous, vertically oriented tube. The tube, which is in a molten state as it exits a die, expands in diameter as it is pulled continuously upward. The diameter stabilizes to a more or less constant value when the tube cools sufficiently to solidify. This solidification occurs a short distance from the die at what is called the frost line. Air cooling systems such as air rings outside of the tube and internal bubble cooling (IBC) systems within the tube are provided close to the exit of the die to ensure that the tube cools quickly enough to remain stable.
After solidifying, the tube passes through stabilizers of various designs and into a flattening device, known as a collapsing frame, to convert the inflated tube into a flattened out film with no air inside. This film is pressed together by motorized nip rolls that continually draw the film upward and away from the extrusion process to form what is call “layflat.” The die and nip roll act as seals, which in steady state, form a trapped, column of air with constant volume inside the tube.
As the film is extruded, thickness variations occur around the circumference of the bubble. It is recognized that these variations are caused by such factors as circumferential nonuniformity in flow distribution channels (ports and spirals) within the die, melt viscosity nonuniformity, and inconsistent annular die gaps through which the polymer exits the die. Additionally, variability of the cooling air and non-uniformity of air aspirated into the cooling air stream from the atmosphere surrounding the extrusion line are major contributors to film thickness variation.
Many film processors rely on conventional blown film equipment to determine the film thickness. This approach typically yields an average variation of +/−10%-20% in film thickness, overall. The presence of such thickness variations creates problems that limit the throughput of downstream conversion equipment, such as printing presses, laminators, or bag machines. In processes where the film is not converted in-line, but is wound onto a roll prior to converting, the thicker and thinner areas of many layers on the roll create hills and valleys on the roll surface, thus deforming the film and magnifying the subsequent converting problem.
A widely practiced method for controlling blown film thickness variation is the use of fans and barriers placed strategically around the process to correct for ambient air variability. This is usually done in combination with manual operator adjustment of the annular die gap through which the liquid polymer melt exits to help minimize the die gap and the effects of variation in the melt viscosity. The main problem with this approach is that the ambient conditions surrounding the process constantly change and require continuous monitoring and barrier and/or fan repositioning. This approach also does not take care of the relatively narrow thickness bands associated with the die ports and spirals, but does allow the processor to use the highest performance equipment available on the market to maximize throughtput on the line.
Thickness improvement over such manual adjustments is found in current systems that actively measure the thickness of the film on-line. Employing closed loop control, these systems use computers to track thickness variations as they occur in the still-inflated bubble and to calculate corrections to individual control zones within the die or cooling systems. These zones impart localized thickness variations which are opposite to those measured and thus to some extent correct for thickness deviations circumferentially around the bubble, including to differing degrees those variations caused by the ports and spirals within the die. Many such systems presently in use require the use of control equipment which improves thickness control but at the expense of throughput rate. A problem associated with all automatic systems is the necessary complexity which creates high cost and requires the use of skilled operators and maintenance personnel.
One approach seeks to control blown film thickness variation by direct mechanical adjustment and deformation of the die lip. In these systems, localized, circumferentially variable, mechanical adjustments to the die lip cause detrimentally large hoop stress and elastic forces to develop in the die lip, thereby resisting deformation and spreading the effect of the adjustment over a larger area than that intended. These problems limit the effective resolution. These systems have correspondingly poor control over thickness variation but do not preclude the use of high performance cooling systems which maximize throughput rate.
Another approach utilizes direct, circumferentially variable, heating of the exit lip from the die. In these systems, individual heaters embedded in the die lip locally heat the lips. Since heat spreads outward in all directions through the steel, the effect is not as locally concentrated as desired and resolution is reduced. Also, heat that is added to the die lip transfers this heat to the molten polymer, thereby raising its local temperature. This extra heat must be removed by the cooling systems, forcing the throughput rate of film production to be lowered.
Yet another approach employs circumferentially variable heating of the cooling air which flows from the primary cooling ring surrounding the blown film bubble. Individual actuators control the local temperature of the cooling air and affect the thickness of the film. Due to the large volumes of air and associated turbulence involved, mixing occurs and significantly degrades the performance of this type of system. Also, heat that is added to the cooling air have the drawback of losing cooling capacity since overall temperature is raised, thus forcing the throughput rate of film production to be lowered.
A more commonly used approach alters in a circumferentially variable way, the flow of air exiting the primary cooling ring surrounding the exterior of the blown film bubble. Individual low pressure actuators mechanically alter the flow of cooling air through associated control zones by using an air blade to section off and bleed air out and away from the air ring which starve feeds the local air flow without causing appreciable pressure drop across the actuator as is described in U.S. Pat. No. 5,281,375. The thickness of the film is affected because more or less heat is removed due to the presence of more or less cooling air. Typically, single flow designs of air rings that use this approach produce acceptable thickness control capability, but have a drawback in that the reduced cooling capability lowers processing throughput rate.
Air blades more recently have been fitted to high performance dual lip air rings, such as those manufactured by Addex, Inc. Multiple radially oriented channels located within the plenum are used to evenly distribute and direct the air flow to the air blades where the low pressure drop air blades section off some of the air that is delivered to the lips the same as with a single flow air ring. This approach has a significant limitation in that there is limited control range capability and can only partially compensate for variations present on a typical blown film process. These systems suffer from the additional disadvantage of added complexity which adds significantly to cost and reduced resolution due to the size of the air blade actuators which limits the ability to control narrow thickness variations commonly present. These systems retain their high throughput capabilities.
A further approach controls in a circumferentially variable way the flow of air exiting the internal bubble cooling (IBC) ring contained within the blown film bubble. This approach does not affect bubble stability since the tube does not lock on the internal cooler and has excellent control of thickness variation. Further, it allows the use of any high performance cooling system exterior to the bubble that is desired and therefore allows for maximum throughput rate. One drawback, however, is that the system cannot be fitted to smaller die diameters, i.e., less than about 10 inches (250 mm), due to space constraints. Additionally, not all processors of film want to employ IBC systems within their process.
It is highly desirable to produce higher quality film during the extrusion process so that the downstream equipment can be run faster and produce better end use products with more consistent thickness while at the same time maximizing the throughput rate of the extrusion line through the use of high performance dual flow air rings and without size restrictions or the requirement to use IBC. It is further desirable to accomplish this using a simpler design to minimize cost and labor.